In situ wet-cell TEM observation of gold nanoparticle motion in an aqueous solution

Author affiliations

1 Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, 200237, People's Republic of China

2 Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai, 200237, People's Republic of China

3 Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA

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Abstract

In situ wet-cell transmission electron microscopy (TEM) technology enables direct observation
of nanomaterials in a fully hydrated environment with high spatial and temporal resolution,
which can be used to address a wide range of scientific problems. In this paper, the
motions of approximately 5-nm sized gold nanoparticles in an aqueous solution are
studied using the wet-cell TEM technology. It is observed that gold nanoparticles
can be either in a single particle or cluster forms, and dynamic displacement and
rotation motions are observed for both forms in the solution. Under electron beam
irradiation, nanoparticles in some clusters gradually fused together; sometimes they
also showed dramatic growth behavior. Mechanisms for the motion and growth of the
particles/clusters are discussed.

Keywords:

Background

Nanoparticle assemblies are often achieved involving liquids. Real-time observation
of nanoparticle assembly and dynamics is thus of great importance
[1,2]. In situ transmission electron microscopy (TEM) techniques provide a local probe of structure
and dynamics that other techniques cannot observe readily. In situ observation provides dynamic information about nanosystems, which is difficult to
obtain by other techniques. Conventional TEM requires drying of samples in order to
be compatible with vacuum. The structural features of the sample can change significantly
during the process. Thus, for samples prepared in liquids, it would be ideal if it
can be observed directly with TEM. With the development of robust silicon nitride
(Si3N4) membrane windows for the in situ cell
[3], the construction of wet-cell and in situ observation of liquids becomes readily possible inside TEM. Applications using the
wet-cell technology are upsurging, and further exciting development is expected in
the future
[1]. For examples, this technique has been applied to the observation of electrochemical
dynamic procedure of Cu
[3] and Ni
[4] nanoclusters, electron beam-induced growth of Pt
[5] and lead sulfide nanocrystals
[6] in liquid, semiconductor nanorod embedded in liquid crystal cells for optoelectronic
applications
[7], Al2O3 nanoparticles and carbon nanotubes in water
[8], and biological cells
[9]. Earlier liquid cell TEM reactor yielded a spatial resolution of only 5nm, but recent
development has improved the resolution to the sub-nanometer range
[5]. The development in Berkeley using graphene sheet to replace Si3N4 even pushed the wet-cell TEM imaging resolution to the atomic level
[10].

Zheng et al. recently made an analysis on gold nanoparticle diffusion during liquid
evaporation
[11]. In addition, Grogan and Bau reported observation with in situ STEM on gold clusters in an aqueous solution using a lower electron beam energy of
20 keV to test the liquid cell hermeticity. However, the image resolution is poorer
due to the lower electron energy
[12].

High spatial resolution TEM requires both high voltage and relatively high electron
beam flux. Local heating and structural transformations may occur during observation
due to the electron beam irradiation
[13]. Previous reports have shown that electron beam can initiate nanoparticle nucleation
and growth in a liquid
[5]. Such beam effect needs to be carefully addressed in wet-cell TEM experiments.

In this paper, we report an in situ observation of gold nanoparticles in aqueous water solution using the wet-cell TEM
technology. Sub-nanometer resolution images were obtained. Dynamic motion and dramatic
growth of clusters of gold nanoparticles have been observed. These observations allow
a discussion of electron beam effect on the growth of nanoparticle clusters.

Methods

Gold nanoparticles are studied with an O-ring sealed clamp on wet-cell developed earlier
at the University of Illinois at Urbana-Champaign
[8]. As shown in Figure
1, the cell utilizes two Si3N4 window grids to confine the liquid. The cell seals via three O-rings that couple
the grids and the top and bottom pieces of the enclosure together. The design utilizes
commercial silicon nitride grids as the substrates and a fixed reusable cell, which
is simply assembled with O-rings and screws, limiting the need for complex microfabrication
procedures to generate appropriate windows for each experiment. Once the grids are
put in place and the liquid is loaded, the cell can be completely assembled within
few minutes. Gold nanoparticles (5 nm in diameter) dispersed in DI water (Nanocs Inc.,
New York, USA, GNP0001-5 (20 ml 0.01% Au)) were tested in situ in TEM chamber. Si3N4 window grids (50-nm thick) from Ted Pella, Inc. (CA, USA) were used to sandwich the
liquid in between. A JEOL 2010 LaB6 TEM system (JEOL Ltd., Tokyo, Japan) was used for the observation, which operated
with a 200-kV electron acceleration voltage.

Results and discussion

Figure
2 shows a cluster of gold nanoparticles observed in an aqueous solution sealed with
the wet-cell. The roughened particle shapes indicate that they are faceted nanocrystals.
Diffraction contrast fringes can be seen in individual particles, confirming the crystalline
structure of the particles. The spatial resolution of the images is about 0.5 nm.
Panels a, b, c, and d of Figure
2 were obtained at 0, 2, 3, and 4 min, respectively. The particles are located at relatively
fixed locations with time, without showing Brownian motion, suggesting they might
be attached on the silicon nitride window without being able to freely move in the
liquid. Changes in the particle morphology occurred over time. In the figure, arrow
1 points to locations where particles coalesce with time. Arrow 2 points to an overlap
region with darker contrast due to 3D arrangement of the nanoparticles. From Figure
2a,b,c, we see that overlapped region is reducing in size with time, suggesting that
the two particles are in different planes and are moving apart from each other. Arrow
3 points to the bottom contour of the moving down particle. From Figure
2a,b,c, we see that the contour is changing shape with time from a flat bottom line
to a relatively rounder one, suggesting the particle was rotating as it moves down.

The above changes in the gold particles do not have to happen in a liquid. When we
put gold nanoparticles on a dry Si3N4 grid, similar behavior was observed. It has also been reported earlier in TEM analysis
that for nanoparticles, coalescing, attachment, rotation, and agglomeration on a dry
substrate can happen at temperatures much lower than the melting temperature of a
bulk material
[14]. For verification of the particle dynamics in solution, further observations were
made. As shown in Figure
3, constant motions of gold particles and clusters in the fluid were seen. Panels a,
b, and c of Figure
3 were taken at 0, 2, and 3 min, respectively. We see that the gold cluster in the
middle changes angle with time, and there are individual dots that are moving constantly.
The center cluster and the individual dots are not in the same depth in the cell.
The cluster has a point fixed on the grid and thus can only rotate with time, but
not move in location. By adjusting the focus of the TEM, we found that the individual
dots are at a different focus depth and thus should be deeper in the liquid. Eight
dots are labeled out which we used to track the motion with time. From the figure,
we see the dots are moving around while changing relative positions to each other
with time. Dots 3 and 4 are moving apart with time, while dots 7 and 8 moved together.
Dot 3 is changing shape because it is a two-particle cluster (see inset of Figure
3b) which is rotating while moving in the liquid. The actual motions of the particles
and clusters in the wet-cell are more dramatic than we directly saw in the figure.
The big cluster was observed to frequently change angle with time and rotate from
one end to the other (approximately 30°) within a fraction of a second; there are
also particles and clusters that move much faster than the eight labeled dots. The
fact that the big cluster can rotate quickly suggests that the individual dots could
also move much quicker if freely suspend in the liquid. The moderate displacement
observed on these dots suggests that they might have been slowed down due to the interaction
from the silicon nitride window. Actually, when we adjust focus on these dots, we
found that they are on about the same focus plane, supporting that they should be
close to a window plane instead of freely suspended in different depths in the liquid.
It has been estimated by White et al.
[15] that, for Brownian motion of 4-nm diameter particles in water, a mean square displacement
<x2>1/2 of approximately 10 μm is expected within a 1/3s drift time, which is much larger
than a regular TEM imaging width. Thus, the fact that the particles remain visible
for more than one frame during the TEM observation indicates that the Brownian motion
is greatly suppressed by the interaction such as that from the window. The observation
shown in Figure
2 can be viewed as an extreme case. When the particles are tightly attached to a window,
their motion in the liquid almost stopped.

Figure 3.A gold cluster rotating in liquid, with particles roaming randomly in the background. (a) 0, (b) 2, and (c) 3 min;(d) Magnification of partial of the cluster from (a), (b), and (c). Inset of (b) is an image focusing on the background particles, showing 3 is a two particle cluster.

Figure
3d is the enlargement of one section of the center cluster, in which we see individual
gold nanoparticles in the cluster are merging with time, confirming that the particle
agglomeration behavior is happening in the liquid environment. The cluster looks like
individual beads that are attached to each other in the beginning but became more
like a web made up of nanowires after 3 min of electron beam irradiation. It is significant
that no bubbling in the liquid is seen during the observation, which indicates that
the welding procedure is happening even below the boiling temperature of water. It
has been calculated that the temperature change in a wet-cell under electron beam
irradiation is generally much smaller than 1 K
[15]. Dramatic changes in gold nanoparticles such as dissolution had been previously observed
under relatively high electron doses of 8 × 105 e/nm2[16]. In our experiment, with the longer observation time of several minutes, the electron
dose may accumulate to beyond a limit that affects the observable gold crystal morphology.

A more dramatic shape change in gold nanoparticle cluster is recorded as shown in
Figure
4. The inset of Figure
4a shows a six-particle cluster during image adjustment. A bubble has been seen nearby,
confirming the cluster is submerged in the liquid. Similar to Figure
2, this cluster is relatively fixed to a location, without making quick rotations or
floating around to large distances. However, in Figure
4a, we see that after the image adjustment, the particles not only fully merged together,
but also changed shape significantly, with some short nanowires growing up in between.
Figure
4b,c,d shows that as time passes (3, 8, and 12 min), the cluster continues to grow
up. Arrow 1 points to locations where new branches grow out from the cluster. Arrow
2 points to where the nanowire bends during growth. Arrow 3 points to the tip of the
cluster which becomes blurred with time; this could be due to the bending of the nanocluster
into the liquid, thus moving out of the focus of the microscope.

Figure 4.A nanocluster that grows dramatically in size with time (a) 0, (b) 3, (c) 8, and (d)
12 min: Arrow 1, new branches grow out; 2, nanowire in the middle of the cluster bends
up and grows longer; 3, a branch on the nanocluster became less clear due to motion
in the water or drifting into different depth of the liquid. Inset of (a) is the nanocluster micrograph that was taken before image adjustment.

Although a longer beam exposure time (12 min in Figure
4 vs. 3 to 4 min in Figures
2 and
3) can be used to partially explain such a dramatic shape change, it is unusual to
see that the cluster increases in size greatly with time. Chemical reactions could
have occurred that transported materials into the cluster from regions away from the
electron beam. Although a noble metal, gold can form many diverse compounds. One possibility
is that gold reacted with water under the high energy electron bombardment and formed
gold oxide or gold hydroxide; however, this growth mechanism is not fully sustainable.
After all gold atoms in the cluster are oxidized, the volume increase will stop. As
the cluster in Figure
4 is fixed in location without being able to move freely around in the liquid, it must
be closely attached to the silicon nitride window, and wetting of the gold on the
window might also account for the morphology change. Arrow 4 in Figure
4d points to a black line in the growing cluster, suggesting a trace for material migration
during the growth. Compared with Figure
4c, it appears that the round dark region near the top center (region 5 in Figure
4d) is the source region for the mass redistribution, and the growing branches to the
left are the regions where the materials are transported toward. The nanowires are
in much lighter color in these TEM bright field images than the earlier gold particles,
suggesting that they are much thinner, thus results in a relatively large area increase.
This supports the idea that the gold wets the window under electron beam irradiation.
In comparison, we irradiated gold nanoclusters on a dry grid for the same amount of
time. Besides the simpler merging behavior like in Figure
2, no such dramatic growth behavior was observed, suggesting that water might have
played a role in helping catalyze the nanowire growth. Recently, Zheng et al. reported
in situ TEM observation of Pt3Fe nanorod growth in solution
[17], in which Pt3Fe nanoparticles attach and coalesce into nanoparticle chains. The chains were winding
and markedly flexible, and gradually turned into nanowires through mass redistribution
procedure. Most of the nanowires remain polystalline and twisted for an extended period
of time. Yuk et al.
[10] further reported in situ TEM observation of Pt nanoparticles coalescing in liquid. These observations are
similar to our results here. It is not fully excluded that there is the possibility
that gold dissolution happened somewhere in the liquid, which got redeposited onto
the growing cluster under the beam interaction. Further study on the gold morphology
change under electron beam irradiation in solution is still needed.

Conclusions

In summary, we report the observation of gold nanoparticles in water solution using
in situ wet-cell TEM technology. The gold nanoparticle system showed a variety of dynamic
behaviors in the aqueous solution involving single particles, particle clusters, and
nanowires, which include dynamic displacement and rotation motions, fusion of particles,
and even dramatic size growth behavior under the electron beam. The fusion and dramatic
growth of the particles happened at temperatures much lower than the gold melting
temperature. Random motions of the gold particles and clusters are greatly suppressed
by the drag from the silicon nitride windows.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

XC carried out the in situ TEM studies, participated in the wet-cell TEM technique advancement, and drafted
the manuscript. JW made substantial contribution to TEM wet-cell development and participated
in the TEM study. All authors read and approved the final manuscript.

Authors' information

XC is currently a professor at the School of Materials Science and Engineering, East
China University of Science and Technology, Shanghai, China. JW is a materials scientist
at Electron Microscopy Center and Materials Science Division, Argonne National Laboratory,
Argonne, IL, USA

Acknowledgments

The TEM experiment was carried out in part in the Frederick Seitz Materials Research
Laboratory Central Facilities, University of Illinois, which are partially supported
by the US Department of Energy under grants DE-FG02-07ER46453 and DE-FG02-07ER46471.
The authors thank SJ Dillon, JM Zuo, CH Lei, W Swiech, and B Sankaran for the kind
support, and Dr. L Martin for the valuable discussions. The support from Shanghai
Leading Academic Discipline Project (B502), Shanghai Key Laboratory Project (08DZ2230500),
and Science and Technology Commission of Shanghai Municipality Project (11nm0507000)
is highly acknowledged. The research was partially accomplished at the Electron Microscopy
Center at Argonne National Laboratory, a US Department of Energy Office of Science
Laboratory operated under contract no. DE-AC02-06CH11357 by UChicago Argonne, LLC.